The present invention relates to an oil level sensor for detecting the level of lubricating oil in an internal combustion engine.
In an industrial or a utility engine, if the engine is operated without checking a shortage of the lubricating oil, the engine is damaged.
Consequently, an oil level sensor is provided for monitoring the level of the lubricating oil while operating. When the oil sensor detects a level of the oil lower than a set level, the engine is stopped or an alarm lamp is turned on to prevent the engine from becoming damaged.
Japanese Patent Applications Laid-Open 62-93417 and 62-26379 disclose oil sensors employed with an oil pressure switch and a reed switch, respectively.
The oil sensor employed with the reed switch consists of various components such as a float and a magnet in addition to the reed switch, so that the oil sensor is complicated in construction. Further, the sensor must be assembled in a crankcase before assembling the engine.
The oil sensor employed with the oil pressure switch can not be used for an engine without an oil pump.
To the contrary, a fluid sensor for detecting an amount of fluid by measuring a resistance of the fluid is known.
The fluid sensor comprises a center electrode and an outer electrode surrounding the center electrode at a predetermined distance. The fluid sensor detects the volume and concentration of the fluid, and other items in accordance with variations of the resistance between the center electrode and the outer electrode.
The fluid sensor is made of a metallic material having a high electric conductivity in order to detect the variation of the resistance.
The sensor body is made by die casting. It is necessary to provide a draft taper for forming a cylindrical space in the sensor body in which the center electrode is inserted. As a result, the cylindrical space must be tapered, so that the distance between the center electrode and an inside wall of the outer electrode changes along the entire effective length of the electrodes. The resistance between both of the electrodes changes accordingly. Therefore, the fluid sensor can not detect the amount of oil with accuracy.
During operation of the engine, the oil in an oil pan is stirred to bubbling. If the distance between the electrodes is large because of a large draft taper, bubbles enter the space between the electrodes, which reduces the sensitivity of the sensor. Consequently, the sensor may determine that the oil pan is empty in spite of having sufficient oil. If the electrode distance is small, the oil adheres to the electrodes because of the viscosity of the oil. Thus, the amount of the oil can not be detected.
Since the die casting has a high surface roughness, it is necessary to finish the surface of the inside wall of the sensor body, which increases the manufacturing process and cost.